1. Field of the Invention
The present invention relates to a melt-kneading method for rubber, elastomer, thermoplastic resin or thermosetting resin whereby a filling material constituted by a nano-level filler is uniformly dispersed in a rubber, elastomer, thermoplastic resin or thermosetting resin. It also relates to a melt-kneaded product of rubber, elastomer, thermoplastic resin or thermosetting resin produced by uniformly dispersing a filling material constituted by a nano-level filler in a rubber, elastomer, thermoplastic resin or thermosetting resin, as well as a molded product constituted by such melt-kneaded product.
2. Description of the Related Art
Production methods for melt-kneaded products in which nano-level fillers such as fine silica grains, single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), carbon fibers (CF), carbon blacks (CB) and clay are uniformly dispersed, as well as melt-kneaded products obtained by these production methods, are already known, and attempts to produce molded resin products by molding these melt-kneaded products have been made. However, examination of the characteristics of obtained molded resin products finds that they are not as good as expected and do not necessarily provide satisfactory results.
Elastomers and rubbers that are high-molecular substances exhibiting rubber elasticity at room temperature are used in a wide range of fields and broad industries such as interior parts of automobiles, construction members, packing and container materials, and medical parts, among others. Essentially, elastomers and rubbers have such properties as low elastic modulus and high elongation at break. It has been known that by adding fillers and reinforcement fibers to elastomers and rubbers, the characteristics of fillers and reinforcement fibers can be added to elastomers and rubbers and also elastomers and rubbers having a high elastic modulus, a characteristic not found in elastomers and rubbers, can be produced.
By mixing these two groups of substances, the functions of the material substances can be demonstrated fully while new characteristics not inherent in each material substance can be added, and resulting materials can meet diverse needs of industries. Accordingly, every time such application is implemented, an elastomer offering appropriate performance values is selected from a wide range of elastomers.
On the other hand, thermoplastic resins and thermosetting resins inherently have a high modulus of elasticity and low elongation at break. Resin complexes having the characteristics of fillers, etc., have been manufactured by adding fillers to thermoplastic resins or thermosetting resins to demonstrate the functions of fillers while making use of the characteristics of thermoplastic resins and thermosetting resins.
In these cases, it has been suggested that it is important to add a filling material, such as a filler, to a rubber, elastomer, thermoplastic resin or thermosetting resin and disperse the filler, etc., both uniformly and at nano level. However, achieving such uniform dispersion is technically difficult. In particularly, uniformly dispersing nano-level fillers is believed to involve a high degree of technical difficulty.
Nano-size level fillers, such as carbon nanotubes, clay (layer silicate), fine silica grains and cage polysilsesquioxane compounds, have an extremely strong cohesive force among filler grains because the grain sizes and void ratios of primary grains are small and it is difficult to remove this cohesive force using normal methods.
For example, we can examine conventional methods using specific examples of carbon nanotubes.
With carbon nanotubes, which are known as nano-level substances, prevention of cohesion is given utmost priority. Traditional methods, therefore, have been to prevent cohesion first and then produce a stable dispersion liquid, and finally mix this dispersion liquid entirely with a high-molecular material matrix to disperse carbon nanotubes. Specific examples are described below.
Methods to physically mix carbon nanotubes in a solution with a polymer composition (Non-patent Literature 1, Appl. Phys, Lett., 1999; 75; 1329, etc.), as well as methods to dissolve carbon nanotubes in a molten polymer composition (Non-patent Literature 2, Chem. Phys. Lett., 2000; 330; 219, etc.), have been attempted. In applications where a filler is added to a resin, naturally methods to melt the resin, etc., add the filler, and then knead them using a knead extruder, etc., are known. For example, known methods include those where a semi-conductive shielding plate is formed using a material made by mixing and dispersing carbon nanotubes in a polyethylene, polypropylene or mixture thereof, wherein a conventional extruder is used to mix the carbon nanotubes (Patent Literature 1, U.S. Pat. No. 4,857,600 and Patent Literature 2, U.S. Pat. No. 5,575,965); and those where a molded resin product whose main ingredient is a resin or elastomer contains nano-scale carbon nanotubes and, if the aforementioned resin is a thermoplastic resin, its melt index (MI) is identified, while the Williams plasticity number is identified if an elastomer is used (Patent Literature 3, Japanese Patent Laid-open No. 2005-88767; Patent Literature 4, Japanese Patent Laid-open No. 2004-338327; and Patent Literature 5, Japanese Patent Laid-open No. 2005-314019); among others.
However, as mentioned earlier nano-size level fillers, or specifically carbon nanotubes, clay (layer silicate), fine silica grains and cage polysilsesquioxane compounds, have an extremely strong cohesive force among filler grains because the grain sizes and void ratios of primary grains are small and it is difficult to remove this cohesive force using the aforementioned methods. In other words, the aforementioned methods have not been able to solve the fundamental problems.
Since the aforementioned methods cannot provide definite solutions, dispersing these nano-size level fillers in elastomers or resins normally requires adding a surface active agent or chemically modifying a filler to increase the affinity between the filler and resin. Methods to do this are already proposed.
Various methods are known for preparing a carbon nanotube dispersion liquid, including the following:
1) A method to disperse carbon nanotubes using an ultrasonic dispersion means is known (Non-patent Literature 3, Langumuir, 2004; 20; 10367). Also, there is a method to produce a water-soluble, single-walled carbon nanotube by introducing a substituent group containing ammonium ions to the pyrene molecule and then applying ultrasonic treatment to the pyrene molecule together with a single-walled carbon nanotube in water to cause the pyrene molecule to be adsorbed to the single-walled carbon nanotube in a non-covalent-bonding manner, by utilizing the fact that the pyrene molecule adsorbs to the carbon nanotube surface due to strong interaction (Non-patent Literature 4, Chem. Lett., 638 (2002)).
2) Methods are known for introducing a hydrophilic function group to the surface of a carbon nanotube by means of acid treatment to improve the dispersibility of the carbon nanotube in various solvents, and then mixing a dispersion liquid with a polymer solution to produce a composite. For example, a method to disperse a single-walled carbon nanotube in a strong acid by means of ultrasonic treatment (Non-patent Literature 5, Science, 280, 1253 (1998)), as well as a method to convert a carboxylic acid group into an acid chloride and then cause the acid chloride to react with an amine compound, after which a long-chain alkyl group is introduced to obtain a product soluble in solvents, by focusing on the fact that single-walled carbon nanotubes have open ends and terminated by oxygen-containing function groups such as carboxylic acid groups (Non-patent Literature 6, Science, 282, 95 (1998)), are known.
3) There is a method to disperse carbon nanotubes in various solvents by coating them with surface active agents and other specific polymers that adsorb to carbon nanotubes (Non-patent Literature 7, Nano Lett., 2003; 3; 269).
However, these methods require complicated operations because the process is implemented in a strong acid, and the effect on dispersion is not sufficient. Also in certain situations such as when a long-chain alkyl group is introduced, some problems emerge including damaged graphene sheet structure of carbon nanotube and affected characteristics of the carbon nanotube itself.
Carbon-nanotube-containing compositions containing a (a) conductive polymer, (b) solvent, (c) carbon nanotube, and if necessary, (d) high-molecular compound, (e) basic compound, (f) surface active agent, (g) silane coupling agent and/or (h) colloidal silica, complexes having a coating film constituted by any of the aforementioned compositions, as well as production methods thereof, are known (Patent Literature 6, Japanese Patent Laid-open No. 2005-97499 and Patent Literature 7, Japanese Patent Laid-open No. Hei 7-102112). These methods allow for uniform dispersion using a solvent and silane coupling agent. However, use of many additive constituents inevitably makes their operation complicated. Although these methods are expected to achieve extremely favorable characteristics for polyimide and other high-molecular compounds, nanocomposites using carbon nanotubes, despite having the above advantages, still present a problem in that dispersing carbon nanotubes uniformly in a resin is extremely difficult because carbon nanotubes become bundled or assume a rope-like form due to their mutual cohesive force (van der Waals force). In particular, smooth surface of carbon nanotubes at atomic level is one factor that reduces the affinity between carbon nanotubes and base material.
Polyimides are generally difficult to dissolve in solvents and when a polyimide is used in a nanocomposite, mixing and dispersing nanograins is difficult. There are methods wherein, by focusing on the fact that polyimides produced by block-copolymerization can be dissolved in solvents, a block-copolymerized polyimide and carbon nanotubes are mixed with a solution produced by dispersing a nonionic surface active agent and/or polyvinyl pyrrolidone (PVP) in an amide polar organic solvent, especially NMP (N-methyl pyrrolidone) and/or dimethyl acetamide (DMAC), or alternatively a polyamic acid, which is a precursor of polyimide, and carbon nanotubes are mixed with a solution produced by dispersing a nonionic surface active agent and/or polyvinyl pyrrolidone (PVP) in an amide polar organic solvent, especially NMP (N-methyl pyrrolidone) and/or dimethyl acetamide (DMAC), in order to obtain a polyamic acid solution in which carbon nanotubes are dispersed, after which the obtained solution is dehydrated to obtain a polyimide in which carbon nanotubes are uniformly dispersed (Patent Literature 9, Japanese Patent Laid-open No. 2006-124613). Here, too, a complicated process is required.
After all, the above methods do not solve the problems mentioned above.
Looking at the aforementioned problems, the inventors feel that, because nano-size level fillers such as carbon nanotubes, clay (layer silicate), fine silica grains and cage polysilsesquioxane compounds have an extremely strong cohesive force among filler grains because the grain sizes and void ratios of primary grains are small, finding a method to remove this cohesive force is the only way to solve the problems. In fact, there is an urgent need to solve these problems.    [Patent Literature 1] U.S. Pat. No. 4,857,600    [Patent Literature 2] U.S. Pat. No. 5,575,965    [Patent Literature 3] Japanese Patent Laid-open No. 2005-88767    [Patent Literature 4] Japanese Patent Laid-open No. 2004-338327    [Patent Literature 5] Japanese Patent Laid-open No. 2005-314019    [Patent Literature 6] Japanese Patent Laid-open No. 2005-97499    [Patent Literature 7] Japanese Patent Laid-open No. Hei 7-102112    [Patent Literature 8] U.S. Pat. No. 5,502,143    [Patent Literature 9] Japanese Patent Laid-open No. 2006-124613    [Non-patent Literature 1] Appl. Phys. Lett., 1999; 75; 1329    [Non-patent Literature 2] Chem. Phys. Lett., 2000; 330; 219    [Non-patent Literature 3] Langumuir, 2004; 20; 10367    [Non-patent Literature 4] Chem. Lett., 638 (2002)    [Non-patent Literature 5] Science, 280, 1253 (1998)    [Non-patent Literature 6] Science, 282, 95 (1998)    [Non-patent Literature 7] Nano Lett., 2003; 3; 269